1. Field of the Invention
This invention relates to portable vibratory seismic signal transducers.
2. The Prior Art
In a widespread method of seismic exploration a continuous wave seismic signal is injected into the earth at designated locations. The continuous waves, which preferably are comprised of a swept frequency wave train, are generated by massive vibrators or transducers mounted on a vehicle such as a truck, tractor, or buggy. A transducer consists essentially of a heavy base plate for contacting the earth's surface, a reaction or inertia mass weighing up to several tons, and a hydraulically-driven linear actuator. The linear actuator shakes the reaction mass relative to the base plate at a desired frequency, usually from 5 to 80 Hertz. The equal and opposite reaction force vibrates the base plate at a corresponding frequency, shaking the earth to generate the desired seismic signal.
Traditionally the base plate has been constructed from rows of I-beams or channels that are welded together at the flanges to form a massive rectangular plate about three feet wide and seven feet long. In such a method of construction, the welds are directly opposite each other at the top and bottom, between adjacent I-beams. Not only does this construction method permit undesired, symmetrical planes of weakness, the welds themselves tend to be imperfect because it is impossible to gain access to the underside of the weld after the I-beams have been positioned side-by-side.
A typical base plate is described in col. 4, lines 11-15 of U.S. Pat. No. 3,306,391 the base plate is shown as part number 34 in FIGS. 1, 2, and particularly FIG. 3 of the 3,306,391 patent which is incorporated by reference into this application as an illustrative example of one embodiment of a seismic signal transducer apparatus. All of the base plates described to or known by applicants have employed the above described base plate. Because of the parallel planes of weakness, prior-art base plates fail prematurely in service.